The present invention relates generally to the field of automatic transmission systems and, more particularly, to a pinless accumulator piston for use in General Motors 4L60, 4T60, 4T60E, 4L60E and 4L80E transmissions (hereinafter “GM transmissions”).
Automatic transmission systems of the prior art have a hydraulic circuit subsystem which includes at least a hydraulic pump, a valve body having fluid conducting passages or circuits, input and exhaust ports formed within the fluid circuits, and a plurality of valves comprised of generally cylindrical pistons having control diameters or lands formed thereon, which alternately open and close the ports to the fluid circuits to regulate the flow and pressure of automatic transmission fluid (hereinafter “ATF”).
Once released into a specific fluid circuit, the pressurized ATF functions to actuate hydraulic clutches, servo pistons, and other components of the transmission. It will be understood that in describing hydraulic circuits, ATF usually changes names when it passes through an orifice or control valve in a specific fluid circuit.
In the GM transmissions accumulators are used to control shift feel during hydraulic clutch application. An accumulator is a spring-loaded device that absorbs a certain amount of apply fluid pressure to cushion the application of a clutch or band against fluid shock. The apply fluid pressure is directed to an accumulator piston that opposes a spring force in the manner of a shock absorber.
In the original equipment manufacture (hereinafter “OEM”) of the GM transmissions, an aluminum accumulator piston typically reciprocates against spring pressure on a cylindrical steel accumulator pin, which eventually causes wear at the accumulator pin/piston interface. Such wear causes hydraulic leakage at the accumulator pin/piston interface resulting in poor shift quality and damaged clutches and bands.
Although accumulator pistons without such an accumulator pin are known, such prior art accumulator pistons invariably have an axial length, which exceeds the piston diameter or in some cases have a stepped design for engaging multiple bores in the valve body and, thus, are stable in operation and do not require a center pin to support their axial movement.
There are several known prior art patents that are available in the field and their discussion follows. One example of a prior art accumulator piston is disclosed in U.S. Pat. No. 4,601,233 to Sugano, which teaches a hydraulic servo device with a built-in accumulator used to dampen a rise in hydraulic fluid pressure. This patent shows an accumulator piston 14, which is fit into the inner diameter portion 16 of a servo piston 12. The servo piston 12 is connected to a central stem 18, which guides the axial travel of the accumulator piston 14.
Another example is shown in U.S. Pat. No. 4,867,294 to de Tuesta, which discloses a stepped accumulator piston 24 that engages two separate cylindrical bores 20, 22 to stabilize axial travel of the piston (FIG. 1).
Another example of an accumulator device is disclosed in U.S. Pat. No. 3,985,063 to Lemon, which teaches an accumulator structure for use in a hydraulic control system for controlling a hydraulic motor. This accumulator piston 50 (FIG. 1) engages multiple bores 56, 58 to stabilize axial travel of the piston and also has an axial length, which exceeds its diameter.
U.S. Pat. No. 5,025,823 to Stevenson and U.S. Pat. No. 4,046,162 to Rodeghiero also disclose accumulator pistons having axial length to diameter ratios, which are not practical for the present application.
Thus, the present invention provides a pinless accumulator piston having an axial length, which is substantially less than the piston diameter as is required in the GM transmission applications and which is stable in operation.